Color display device

ABSTRACT

The present invention provides a color display device in which each pixel can display four high quality color states. More specifically, an electrophoretic fluid is provided which comprises four types of particles, dispersed in a solvent or solvent mixture. The fluid may further comprise substantially uncharged neutral buoyancy particles.

This application claims the benefit of U.S. Provisional Application Nos.61/824,887, filed May 17, 2013; 61/893,831, filed Oct. 21, 2013; and61/974,858, filed Apr. 3, 2014. The contents of the above-identifiedapplications are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention is directed to a color display device in whicheach pixel can display four high quality color states, anelectrophoretic fluid for such an electrophoretic display and drivingmethods for such a display device.

BACKGROUND OF THE INVENTION

In order to achieve a color display, color filters are often used. Themost common approach is to add color filters on top of black/whitesub-pixels of a pixellated display to display the red, green and bluecolors. When a red color is desired, the green and blue sub-pixels areturned to the black state so that the only color displayed is red. Whenthe black state is desired, all three-sub-pixels are turned to the blackstate. When the white state is desired, the three sub-pixels are turnedto red, green and blue, respectively, and as a result, a white state isseen by the viewer.

The biggest disadvantage of such a technique is that since each of thesub-pixels has a reflectance of about one third of the desired whitestate, the white state is fairly dim. To compensate this, a fourthsub-pixel may be added which can display only the black and whitestates, so that the white level is doubled at the expense of the red,green or blue color level (where each sub-pixel is only one fourth ofthe area of the pixel). Brighter colors can be achieved by adding lightfrom the white pixel, but this is achieved at the expense of color gamutto cause the colors to be very light and unsaturated. A similar resultcan be achieved by reducing the color saturation of the threesub-pixels. Even with this approach, the white level is normallysubstantially less than half of that of a black and white display,rendering it an unacceptable choice for display devices, such ase-readers or displays that need well readable black-white brightness andcontrast.

SUMMARY OF THE INVENTION

One aspect of the present invention is directed to a display layercomprising an electrophoretic medium and having first and secondsurfaces on opposed sides thereof, the electrophoretic medium comprisinga first type of positive particles, a first type of negative particles,a second type of positive particles and a second type of negativeparticles, all dispersed in a solvent or solvent mixture, the four typeof particles having respectively optical characteristics differing fromone another, such that:

(a) application of an electric field which has the same polarity as thefirst type of positive particles will cause the optical characteristicsof the first type of positive particles to be displayed at the firstsurface; or

(b) application of an electric field which has the same polarity as thefirst type of negative particles will cause the optical characteristicof the first type of negative particles to be displayed at the firstsurface; or

(c) once the optical characteristic of the first type of positiveparticles is displayed at the first surface, application of an electricfield which has the same polarity as the second type of negativeparticles, but is not strong enough to overcome the attraction forcebetween the first type of positive particles and the first type ofnegative particles, but sufficient to overcome the attraction forcesbetween other oppositely charged particles will cause the opticalcharacteristic of the second type of negative particles to be displayedat the first surface; or

(d) once the optical characteristic of the first type of negativeparticles is displayed at the first surface, application of an electricfield which has the same polarity as the second type of positiveparticles, but is not strong enough to overcome the attraction forcebetween the first type of positive particles and the first type ofnegative particles, but sufficient to overcome the attraction forcesbetween other oppositely charged particles will cause the opticalcharacteristic of the second type of positive particles to be displayedat the first surface.

In one embodiment, the first type of positive particles is blackparticles, the first type of negative particles is yellow particles, thesecond type of positive particles is the red particles and the secondtype of negative particles is the white particles.

In one embodiment, the charges of the first type of positive particlesand the first type of negative particles are higher than the second typeof positive particles and the second type of negative particles.

In one embodiment, the charges of the lower charged particles are lessthan 50% of the charges of the higher charged particles. In oneembodiment, the charges of the lower charged particles are 5% to 30% ofthe charges of the higher charged particles. In one embodiment, thecharges of the lower charged particles are less than 75% of the chargesof the higher charged particles. In one embodiment, the charges of thelower charged particles are 15% to 55% of the charges of the highercharged particles.

In one embodiment, the electrophoretic medium further comprisingsubstantially uncharged neutral buoyancy particles. In one embodiment,the neutral buoyancy particles are non-charged.

Another aspect of the present invention is directed to a driving methodfor an electrophoretic fluid comprising four types of charged pigmentparticles dispersed in a solvent or solvent mixture, wherein the fourtypes of charged pigment particles are high positive charged pigmentparticles, high negative charged pigment particles, low positive chargedpigment particles and low negative charged particles, which methodcomprises

(a) driving a pixel to the color state of one of the low chargedparticles; followed by

(b) driving the pixel to the color state of high charged particles,wherein the low charged particles and the high charged particles carryopposite charge polarities.

In one embodiment, the method further comprises a shaking waveform.

In one embodiment of the driving method, the high positive chargedparticles are black. In another embodiment, the high negative chargedparticles are yellow. In a further embodiment, the low positive chargedparticles are red. In yet a further embodiment, the low negative chargedparticles are white.

A further aspect of the invention is directed to a driving method for anelectrophoretic fluid comprising four types of charged pigment particlesdispersed in a solvent or solvent mixture, wherein the four types ofcharged pigment particles are high positive charged pigment particles,high negative charged pigment particles, low positive charged pigmentparticles and low negative charged particles, which method comprises

-   -   (a) applying a shaking waveform;    -   (b) applying a high driving voltage having the same polarity as        one type of high charged pigment particles to drive to a color        state of the high charged pigment particles;    -   (c) applying a low driving voltage having the same polarity as        one type of low charged pigment particles to drive to a color        state of low charged pigment particles; and    -   (d) applying a high driving voltage having the same polarity as        the high charged pigment particles to drive to a color state of        the high charged pigment particles;        wherein the high charged pigment particles and the low charged        pigment particles are oppositely charged and the driving method        is DC balanced.

In yet a further aspect of the invention is directed to anelectrophoretic fluid comprising four types of charged pigment particlesdispersed in a solvent or solvent mixture, wherein the four types ofcharged pigment particles are high positive charged pigment particles,high negative charged pigment particles, low positive charged pigmentparticles and low negative charged particles and the low chargedparticles have a charge intensity which is less than 75% of the chargeintensity of the high charged particles.

In one embodiment, the low positive charged particles have a chargeintensity which is less than 50% of the charge intensity of the highpositive charged particles and the low negative charged particles have acharge intensity which is less than 75% of the charge intensity of thehigh negative charged particles.

In one embodiment, the fluid further comprises substantially unchargedneutral buoyancy particles, which may be non-charged.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a display layer which can display four different colorstates.

FIGS. 2-1 to 2-3 illustrate an example of the present invention.

FIG. 3 demonstrates display cells unaligned with pixel electrodes.

FIGS. 4A and 4B illustrate driving methods of the present invention.

FIG. 5 shows a shaking waveform which may be incorporated into drivingsequences.

FIGS. 6A and 6B show example waveforms for driving the display device ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The electrophoretic fluid of present invention comprises two pairs ofoppositely charged particles. The first pair consists of a first type ofpositive particles and a first type of negative particles and the secondpair consists of a second type of positive particles and a second typeof negative particles.

In the two pairs of oppositely charged particles, one pair carries astronger charge than the other pair. Therefore the four types ofparticles may also be referred to as high positive particles, highnegative particles, low positive particles and low negative particles.

As an example shown in FIG. 1, the black particles (K) and yellowparticles (Y) are the first pair of oppositely charged particles, and inthis pair, the black particles are the high positive particles and theyellow particles are the high negative particles. The red particles (R)and the white particles (W) are the second pair of oppositely chargedparticles and in this pair, the red particles are the low positiveparticles and the white particles are the low negative particles.

In another example not shown, the black particles may be the highpositive particles; the yellow particles may be the low positiveparticles; the white particles may be the low negative particles and thered particles may be the high negative particles.

In addition, the color states of the four types of particles may beintentionally mixed. For example, because yellow pigment by nature oftenhas some greenish tint and if a better yellow color state is desired,yellow particles and red particles may be used where both types ofparticles carry the same charge polarity and the yellow particles arehigher charged than the red particles. As a result, at the yellow state,there will be a small amount of the red particles mixed with thegreenish yellow particles to cause the yellow state to have better colorpurity.

It is understood that the scope of the invention broadly encompassesparticles of any colors as long as the four types of particles havevisually distinguishable colors.

For the white particles, they may be formed from an inorganic pigment,such as TiO₂, ZrO₂, ZnO, Al₂O₃ Sb₂O₃, BaSO₄, PbSO₄ or the like.

For the black particles, they may be formed from CI pigment black 26 or28 or the like (e.g., manganese ferrite black spinel or copper chromiteblack spinel) or carbon black.

Particles of other colors are independently of a color such as red,green, blue, magenta, cyan or yellow. The pigments for color particlesmay include, but are not limited to, CI pigment PR 254, PR122, PR149,PG36, PG58, PG7, PB28, PB15:3, PY83, PY138, PY150, PY155 or PY20. Thoseare commonly used organic pigments described in color index handbooks,“New Pigment Application Technology” (CMC Publishing Co, Ltd, 1986) and“Printing Ink Technology” (CMC Publishing Co, Ltd, 1984). Specificexamples include Clariant Hostaperm Red D3G 70-EDS, Hostaperm PinkE-EDS, PV fast red D3G, Hostaperm red D3G 70, Hostaperm Blue B2G-EDS,Hostaperm Yellow H4G-EDS, Novoperm Yellow HR-70-EDS, Hostaperm GreenGNX, BASF Irgazine red L 3630, Cinquasia Red L 4100 HD, and Irgazin RedL 3660 HD; Sun Chemical phthalocyanine blue, phthalocyanine green,diarylide yellow or diarylide AAOT yellow.

The color particles may also be inorganic pigments, such as red, green,blue and yellow. Examples may include, but are not limited to, CIpigment blue 28, CI pigment green 50 and CI pigment yellow 227.

In addition to the colors, the four types of particles may have otherdistinct optical characteristics, such as optical transmission,reflectance, luminescence or, in the case of displays intended formachine reading, pseudo-color in the sense of a change in reflectance ofelectromagnetic wavelengths outside the visible range.

A display layer utilizing the display fluid of the present invention hastwo surfaces, a first surface (13) on the viewing side and a secondsurface (14) on the opposite side of the first surface (13). The displayfluid is sandwiched between the two surfaces. On the side of the firstsurface (13), there is a common electrode (11) which is a transparentelectrode layer (e.g., ITO), spreading over the entire top of thedisplay layer. On the side of the second surface (14), there is anelectrode layer (12) which comprises a plurality of pixel electrodes (12a).

The pixel electrodes are described in U.S. Pat. No. 7,046,228, thecontent of which is incorporated herein by reference in its entirety. Itis noted that while active matrix driving with a thin film transistor(TFT) backplane is mentioned for the layer of pixel electrodes, thescope of the present invention encompasses other types of electrodeaddressing as long as the electrodes serve the desired functions.

Each space between two dotted vertical lines in FIG. 1 denotes a pixel.As shown, each pixel has a corresponding pixel electrode. An electricfield is created for a pixel by the potential difference between avoltage applied to the common electrode and a voltage applied to thecorresponding pixel electrode.

The percentages of the four types of particles in the fluid may vary.For example, in a fluid having black/yellow/red/white particles, theblack particle may take up 0.1% to 10%, preferably 0.5% to 5%, by volumeof the electrophoretic fluid; the yellow particle may take up 1% to 50%,preferably 5% to 15%, by volume of the fluid; and each type of the redand white particles may take up 2% to 20%, preferably 4% to 10%, byvolume of the fluid.

The solvent in which the four types of particles are dispersed is clearand colorless. It preferably has a low viscosity and a dielectricconstant in the range of about 2 to about 30, preferably about 2 toabout 15 for high particle mobility. Examples of suitable dielectricsolvent include hydrocarbons such as isopar, decahydronaphthalene(DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oil, siliconfluids, aromatic hydrocarbons such as toluene, xylene,phenylxylylethane, dodecylbenzene or alkylnaphthalene, halogenatedsolvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene,dichlorobenzotrifluoride, 3,4,5-trichlorobenzotri fluoride,chloropentafluoro-benzene, dichlorononane or pentachlorobenzene, andperfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M Company,St. Paul Minn., low molecular weight halogen containing polymers such aspoly(perfluoropropylene oxide) from TCI America, Portland, Oreg.,poly(chlorotrifluoro-ethylene) such as Halocarbon Oils from HalocarbonProduct Corp., River Edge, N.J., perfluoropolyalkylether such as Galdenfrom Ausimont or Krytox Oils and Greases K-Fluid Series from DuPont,Delaware, polydimethylsiloxane based silicone oil from Dow-corning(DC-200).

In one embodiment, the charge carried by the “low charge” particles maybe less than about 50%, preferably about 5% to about 30%, of the chargecarried by the “high charge” particles. In another embodiment, the “lowcharge” particles may be less than about 75%, or about 15% to about 55%,the charge carried by the “high charge” particles. In a furtherembodiment, the comparison of the charge levels as indicated applies totwo types of particles having the same charge polarity.

The charge intensity may be measured in terms of zeta potential. In oneembodiment, the zeta potential is determined by Colloidal DynamicsAcoustoSizer IIM with a CSPU-100 signal processing unit, ESA EN# Attnflow through cell (K:127). The instrument constants, such as density ofthe solvent used in the sample, dielectric constant of the solvent,speed of sound in the solvent, viscosity of the solvent, all of which atthe testing temperature (25° C.) are entered before testing. Pigmentsamples are dispersed in the solvent (which is usually a hydrocarbonfluid having less than 12 carbon atoms), and diluted to between 5-10% byweight. The sample also contains a charge control agent (Solsperse17000®, available from Lubrizol Corporation, a Berkshire Hathawaycompany; “Solsperse” is a Registered Trade Mark), with a weight ratio of1:10 of the charge control agent to the particles. The mass of thediluted sample is determined and the sample is then loaded into the flowthrough cell for determination of the zeta potential.

The magnitudes of the “high positive” particles and the “high negative”particles may be the same or different. Likewise, the magnitudes of the“low positive” particles and the “low negative” particles may be thesame or different.

It is also noted that in the same fluid, the two pairs of high-lowcharge particles may have different levels of charge differentials. Forexample, in one pair, the low positively charged particles may have acharge intensity which is 30% of the charge intensity of the highpositively charged particles and in another pair, the low negativelycharged particles may have a charge intensity which is 50% of the chargeintensity of the high negatively charged particles.

It is also noted that the four types of particles may have differentparticle sizes. For example, the smaller particles may have a size whichranges from about 50 nm to about 800 nm. The larger particles may have asize which is about 2 to about 50 times, and more preferably about 2 toabout 10 times, the sizes of the smaller particles.

The following is an example illustrating the present invention.

EXAMPLE 1

This example is demonstrated in FIG. 2. The high positive particles areof the black color (K); the high negative particles are of a yellowcolor (Y); the low positive particles are of a red color (R); and thelow negative particles are of a white color (W).

In FIG. 2( a), when a high negative voltage potential difference (e.g.,−15V) is applied to a pixel for a time period of sufficient length, anelectric field is generated to cause the yellow particles (Y) to bepushed to the common electrode (21) side and the black particles (K)pulled to the pixel electrode (22 a) side. The red (R) and white (W)particles, because they carry weaker charges, move slower than thehigher charged black and yellow particles and as a result, they stay inthe middle of the pixel, with white particles above the red particles.In this case, a yellow color is seen at the viewing side.

In FIG. 2( b), when a high positive voltage potential difference (e.g.,+15V) is applied to the pixel for a time period of sufficient length, anelectric field of an opposite polarity is generated which causes theparticle distribution to be opposite of that shown in FIG. 2( a) and asa result, a black color is seen at the viewing side.

In FIG. 2( c), when a lower positive voltage potential difference (e.g.,+3V) is applied to the pixel of FIG. 2( a) (that is, driven from theyellow state) for a time period of sufficient length, an electric fieldis generated to cause the yellow particles (Y) to move towards the pixelelectrode (22 a) while the black particles (K) move towards the commonelectrode (21). However, when they meet in the middle of the pixel, theystop moving and remain there because the electric field generated by thelow driving voltage is not strong enough to overcome the strongattraction between them. On the other hand, the electric field generatedby the low driving voltage is sufficient to separate the weaker chargedwhite and red particles to cause the low positive red particles (R) tomove all the way to the common electrode (21) side (i.e., the viewingside) and the low negative white particles (W) to move to the pixelelectrode (22 a) side. As a result, a red color is seen. It is alsonoted that in this figure, there are also attraction forces betweenweaker charged particles (e.g., R) with stronger charged particles ofopposite polarity (e.g., Y). However, these attraction forces are not asstrong as the attraction forces between two types of stronger chargedparticles (K and Y) and therefore they can be overcome by the electricfield generated by the low driving voltage. In other words, weakercharged particles and the stronger charged particles of oppositepolarity can be separated.

In FIG. 2( d), when a lower negative voltage potential difference (e.g.,−3V) is applied to the pixel of FIG. 2( b) (that is, driven from theblack state) for a time period of sufficient length, an electric fieldis generated which causes the black particles (K) to move towards thepixel electrode (22 a) while the yellow particles (Y) move towards thecommon electrode (21). When the black and yellow particles meet in themiddle of the pixel, they stop moving and remain there because theelectric field generated by the low driving voltage is not sufficient toovercome the strong attraction between them. At the same time, theelectric field generated by the low driving voltage is sufficient toseparate the white and red particles to cause the low negative whiteparticles (W) to move all the way to the common electrode side (i.e.,the viewing side) and the low positive red particles (R) move to thepixel electrode side. As a result, a white color is seen. It is alsonoted that in this figure, there are also attraction forces betweenweaker charged particles (e.g., W) with stronger charged particles ofopposite polarity (e.g., K). However, these attraction forces are not asstrong as the attraction forces between two types of stronger chargedparticles (K and Y) and therefore they can be overcome by the electricfield generated by the low driving voltage. In other words, weakercharged particles and the stronger charged particles of oppositepolarity can be separated.

Although in this example, the black particles (K) is demonstrated tocarry a high positive charge, the yellow particles (Y) carry a highnegative charge, the red (R) particles carry a low positive charge andthe white particles (W) carry a low negative charge, in practice, theparticles carry a high positive charge, or a high negative charge, or alow positive charge or a low negative charge may be of any colors. Allof these variations are intended to be within the scope of thisapplication.

It is also noted that the lower voltage potential difference applied toreach the color states in FIGS. 2( c) and 2(d) may be about 5% to about50% of the full driving voltage potential difference required to drivethe pixel from the color state of high positive particles to the colorstate of the high negative particles, or vice versa.

The electrophoretic fluid as described above is filled in display cells.The display cells may be microcups as described in U.S. Pat. No.6,930,818, the content of which is incorporated herein by reference inits entirety. The display cells may also be other types ofmicro-containers, such as microcapsules, microchannels or equivalents,regardless of their shapes or sizes. All of these are within the scopeof the present application.

As shown in FIG. 3, the display cells (30), in the present invention,and the pixel electrodes (32 a) do not have to be aligned.

In a further aspect of the present invention, the fluid may furthercomprise substantially uncharged neutral buoyancy particles.

The term “substantially uncharged” refers to the particles which areeither uncharged or carry a charge which is less than 5% of the averagecharge carried by the higher charged particles. In one embodiment, theneutral buoyancy particles are non-charged.

The term “neutral buoyancy” refers to particles which do not rise orfall with gravity. In other words, the particles would float in thefluid between the two electrode plates. In one embodiment, the densityof the neutral buoyancy particles may be the same as the density of thesolvent or solvent mixture in which they are dispersed.

The concentration of the substantially uncharged neutral buoyancyparticles in the display fluid is preferably in the range of about 0.1to about 10% by volume, more preferably in the range of about 0.1 toabout 5% by volume.

The term “about” refers to a range which is ±10% of the indicated value.

The substantially uncharged neutral buoyancy particles may be formedfrom a polymeric material. The polymeric material may be a copolymer ora homopolymer.

Examples of the polymeric material for the substantially unchargedneutral buoyancy particles may include, but are not limited to,polyacrylate, polymethacrylate, polystyrene, polyaniline, polypyrrole,polyphenol and polysiloxane. Specific examples of the polymeric materialmay include, but are not limited to, poly(pentabromophenylmethacrylate), poly(2-vinylnapthalene), poly(naphthyl methacrylate),poly(alpha-methystyrene), poly(N-benzyl methacrylamide) and poly(benzylmethacrylate).

More preferably, the substantially uncharged neutral buoyancy particlesare formed from a polymer which is not soluble in the solvent of thedisplay fluid, and also has a high refractive index. In one embodiment,the refractive index of the substantially uncharged neutral buoyancyparticles is different from that of the solvent or solvent mixture inwhich the particles are dispersed. However, typically the refractiveindex of the substantially uncharged neutral buoyancy particles ishigher than that of the solvent or solvent mixture. In some cases, therefractive index of the substantially uncharged neutral buoyancyparticles may be above 1.45.

In one embodiment, the materials for the substantially uncharged neutralbuoyancy particles may comprise an aromatic moiety.

The substantially uncharged neutral buoyancy particles may be preparedfrom monomers through polymerization techniques, such as suspensionpolymerization, dispersion polymerization, seed polymerization,soap-free polymerization, emulsion polymerization or physical method,including inverse emulsification-evaporation process. The monomers arepolymerized in the presence of a dispersant. The presence of thedispersant allows the polymer particles to be formed in a desired sizerange and the dispersant may also form a layer physically or chemicallybonded to the surface of the polymer particles to prevent the particlesfrom agglomeration.

The dispersant preferably has a long chain (of at least eight atoms),which may stabilize the polymer particles in a hydrocarbon solvent. Suchdispersants may be an acrylate-terminated or vinyl-terminatedmacromolecule, which are suitable because the acrylate or vinyl groupcan co-polymerize with the monomer in the reaction medium.

One specific example of the dispersant is acrylate terminatedpolysiloxane (Gelest, MCR-M17, MCR-M22),

Another type of suitable dispersants is polyethylene macromonomers, asshown below:

CH₃—[—CH₂—]_(n)—CH₂O—C(═O)—C(CH₃)═CH₂

The backbone of the macromonomer may be a polyethylene chain and theinteger “n” may be 30-200. The synthesis of this type of macromonomersmay be found in Seigou Kawaguchi et al, Designed Monomers and Polymers,2000, 3, 263.

If the fluid system is fluorinated, the dispersants are then preferablyalso fluorinated.

Alternatively, the substantially uncharged neutral buoyancy particlesmay also be formed from a core particle coated with a polymeric shelland the shell may be formed, for example, from any of the polymericmaterial identified above.

The core particle may be of an inorganic pigment such as TiO₂, ZrO₂,ZnO, Al₂O—₃, CI pigment black 26 or 28 or the like (e.g., manganeseferrite black spinel or copper chromite black spinel), or an organicpigment such as phthalocyanine blue, phthalocyanine green, diarylideyellow, diarylide AAOT yellow, and quinacridone, azo, rhodamine,perylene pigment series from Sun Chemical, Hansa yellow G particles fromKanto Chemical, and Carbon Lampblack from Fisher, or the like.

In the case of core-shell substantially uncharged neutral buoyancyparticles, they may be formed by a microencapsulation method, such ascoacervation, interfacial polycondensation, interfacial cross-linking,in-suit polymerization or matrix polymerization.

The size of the substantially uncharged neutral buoyancy particles ispreferably in the range of about 100 nanometers to about 5 microns.

In one embodiment of this aspect of the present invention, thesubstantially uncharged neutral buoyancy particles added to the fluidmay have a color substantially the same visually to the color of one ofthe four types of charged particles. For example, in a display fluid,there may be charged black, yellow, red and white particles andsubstantially uncharged neutral buoyancy particles, and in this case,the substantially uncharged neutral buoyancy particles may be black,yellow, red or white.

In another embodiment, the substantially uncharged neutral buoyancyparticles may have a color substantially different from the color ofeither one of the four types of charged particles.

The presence of the substantially uncharged neutral buoyancy particlesin the fluid increases reflection of incident light, thus also improvingthe contrast ratio, especially if they are formed from a reflectivematerial.

The image stability may also be improved by the addition of thesubstantially uncharged neutral buoyancy particles in the four particlefluid system. The substantially uncharged neutral buoyancy particles canfill in the gaps resulted from the charged particles being over packedon the surface of an electrode under an electrical field, thuspreventing the charged particles from settling due to the gravitationalforce.

In addition, if the substantially uncharged neutral buoyancy particlesare white, they may enhance the reflectivity of the display. If they areblack, they may enhance the blackness of the display.

In any case, the substantially uncharged neutral buoyancy particles donot affect the driving behavior of the four types of charged particlesin the fluid.

Ideally when a high positive driving voltage (e.g. +15V) is applied asshown in FIG. 2( b), the electric field generated would cause the highpositive black particles to move towards the common electrode side(i.e., the viewing side) and the high negative yellow particles and thelow negative white particles to move towards the non-viewing side, toshow the black state. The low positive red particles would move towardsthe viewing side. But since the red particles carry a lower chargecompared to the black particles, they move slower and as a result, theblack color is seen at the viewing side. However, in practice, the blackstate achieved may have a reddish tint. This could be caused by some ofthe red particles becoming mixed with the black particles at the viewingside.

The present invention also provides driving methods which can resolvethe unsatisfactory color issue. In one of the driving methods, a pixelis first driven towards the color state of one of the low chargedparticles before being driven towards the color state of high chargedparticles, wherein the low charged particles and the high chargedparticles carry opposite charge polarities.

For example, a pixel may be driven towards the black color state,according to the following steps:

-   -   a) driving first to the color state of the white particles (low        negative charged) by applying a low negative driving voltage;        and    -   b) driving towards the color state of the black particles (high        positive charged) by applying a high positive driving voltage.

This driving sequence is illustrated in FIG. 4A.

In step (a), once at the white state (e.g., FIG. 2( d)), the two typesof “high charged” particles, black and yellow, will attract to eachother to cause them to stay in the middle of the pixel and the lowpositive charged red pigment particles would move to be near or at thepixel electrode.

In step (b), the white and yellow particles are pushed to the pixelelectrode side, and the low positive charged red particles are much lesslikely to show up at the viewing side. This sequence will result in abetter quality of the black state.

In this driving method, a white color state is driven directly towardsthe black state without going through the red or yellow color state. Ithas also been found that higher the quality of the white state in step(a) will lead to a higher quality of the black state in step (b). The“higher quality of the white state” simply means a high L* value and lowa* and b* values in the L*a*b* color system, for the white state.

A similar driving method may be applied to driving a pixel to the yellowstate. The method will have the following steps:

-   -   a) driving first to the color state of the red particles (low        positive charged) by applying a low positive driving voltage;        and    -   b) driving towards the color state of the yellow particles (high        negative charged) by applying a high negative driving voltage.

This driving sequence is shown in FIG. 4B.

In this driving method, a red color state is driven directly towards theyellow state without going through the white or black color state. Ithas also been found that higher the quality of the red state in step (a)will lead to a higher quality of the yellow state in step (b). The“higher quality of the red state” simply means a high a* value in theL*a*b* color system, for the red state.

The driving method shown in FIGS. 4A and 4B may also be summarized asfollows:

A driving method for driving a display layer which comprises anelectrophoretic medium and has first and second surfaces on opposedsides thereof, the electrophoretic medium comprising a first type ofpositive particles, a first type of negative particles, a second type ofpositive particles and a second type of negative particles, alldispersed in a solvent or solvent mixture, the four type of particleshaving respectively optical characteristics differing from one another,which method comprises:

(a) applying an electric field which is not sufficient to overcome theattraction force between the first type of positive particles and thefirst type of negative particles and has the same polarity as the secondtype of positive or negative particles to cause the opticalcharacteristics of the second type of positive or negative particles tobe displayed at the first surface; and

(b) applying an electric field which is sufficient to overcome theattraction force between the first type of positive particles and thefirst type of negative particles and has the polarity opposite of thepolarity of the electric field in step (a) to cause the opticalcharacteristic of the first type of positive particles or the first typeof negative particles to be displayed at the first surface.

In addition, to ensure both color brightness and color purity, a shakingwaveform, prior to driving from one color state to another color state,may be used. The shaking waveform consists of repeating a pair ofopposite driving pulses for many cycles. For example, the shakingwaveform may consist of a +15V pulse for 20 msec and a −15V pulse for 20msec and such a pair of pulses is repeated for 50 times. The total timeof such a shaking waveform would be 2000 msec (see FIG. 5).

In practice, there may be at least 10 repetitions (i.e., ten pairs ofpositive and negative pulses).

The shaking waveform may be applied regardless of the optical state(black, white, red or yellow) prior to a driving voltage is applied.After the shaking waveform is applied, the optical state would not be apure white, pure black, pure yellow or pure red. Instead, the colorstate would be from a mixture of the four types of pigment particles.

Each of the driving pulse in the shaking waveform is applied for notexceeding 50% (or not exceeding 30%, 10% or 5%) of the driving timerequired from the full black state to the full yellow state in theexample. For example, if it takes 300 msec to drive a display devicefrom a full black state to a full yellow state or vice versa, theshaking waveform may consist of positive and negative pulses, eachapplied for not more than 150 msec. In practice, it is preferred thatthe pulses are shorter.

In one embodiment, a shaking waveform may be applied prior to thedriving sequence of FIG. 4A or FIG. 4B.

In another embodiment, a pixel may be:

-   -   (i) applied a shaking waveform;    -   (ii) driven to black (i.e., the first-time black state);    -   (iii) driven to white; and then    -   (iv) driven to black (i.e., the second-time black state).

In this sequence, step (ii) may be carried out according to FIG. 2( b);step (iii) may be carried out according to FIG. 2( d); and step (iv) maybe carried out according to FIG. 4A.

An example waveform for this driving sequence is shown in FIG. 6A. Inany of the driving sequences of the present invention, the waveforms arepreferably DC balanced, that is, the average voltage applied across thedisplay is substantially zero when integrated over a time period. InFIG. 6A, in the initial step as shown, a high negative driving voltageis applied to ensure DC balance of the entire waveform.

Similarly, both the shaking waveform and the method of FIG. 4B may beincorporated into a driving sequence:

-   -   (i) applied a shaking waveform;    -   (ii) driven to yellow (i.e., the first-time yellow state);    -   (iii) driven to red; and then    -   (iv) driven to yellow (i.e., the second-time yellow state).

In this sequence, step (ii) may be carried out according to FIG. 2( a);step (iii) may be carried out according to FIG. 2( c); and step (iv) maybe carried out according to FIG. 4B.

An example waveform for this driving sequence is shown in FIG. 6B, whichis also “DC balanced”.

In practice, the quality of the first-time color state (black or yellow)is usually inferior compared with the second-time color state (black oryellow).

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particularsituation, materials, compositions, processes, process step or steps, tothe objective, spirit and scope of the present invention. All suchmodifications are intended to be within the scope of the claims appendedhereto.

What is claimed is:
 1. An electrophoretic fluid comprising four types ofcharged pigment particles dispersed in a solvent or solvent mixture,wherein the four types of charged pigment particles are high positivecharged pigment particles, high negative charged pigment particles, lowpositive charged pigment particles and low negative charged particlesand the low charged particles have a charge intensity which is less than75% of the charge intensity of the high charged particles.
 2. The fluidof claim 1, wherein the low positive charged particles have a chargeintensity which is less than 50% of the charge intensity of the highpositive charged particles and the low negative charged particles have acharge intensity which is less than 75% of the charge intensity of thehigh negative charged particles.
 3. The fluid of claim 2, wherein thehigh positive charged particles are black particles, the high negativecharged particles are yellow particles, the low positive chargedparticles are red particles and the low negative charged particles arewhite particles.
 4. The fluid of claim 2, further comprisingsubstantially uncharged neutral buoyancy particles.
 5. The fluid ofclaim 4, wherein the substantially uncharged neutral buoyancy particlesare non-charged.
 6. A display layer comprising an electrophoretic mediumand having first and second surfaces on opposed sides thereof, theelectrophoretic medium comprising a first type of positive particles, afirst type of negative particles, a second type of positive particles,and a second type of negative particles, all dispersed in a solvent orsolvent mixture, the four type of particles having respectively opticalcharacteristics differing from one another, such that: (a) applicationto a pixel an electric field having the same polarity as the first typeof positive particles will cause the optical characteristics of thefirst type of positive particles to be displayed at the first surface;or (b) application to a pixel an electric field which has the samepolarity as the first type of negative particles will cause the opticalcharacteristic of the first type of negative particles to be displayedat the first surface; or (c) once the optical characteristic of thefirst type of positive particles is displayed at the first surface,application to a pixel an electric field having the same polarity as thesecond type of negative particles, but not strong enough to overcome theattraction force between the first type of positive particles and thefirst type of negative particles, but sufficient to overcome theattraction forces between other oppositely charged particles will causethe optical characteristic of the second type of negative particles tobe displayed at the first surface; or (d) once the opticalcharacteristic of the first type of negative particles is displayed atthe first surface, application to a pixel an electric field having thesame polarity as the second type of positive particles, but not strongenough to overcome the attraction force between the first type ofpositive particles and the first type of negative particles, butsufficient to overcome the attraction forces between other oppositelycharged particles will cause the optical characteristic of the secondtype of positive particles to be displayed at the first surface.
 7. Thelayer of claim 6, wherein the first type of positive particles is blackparticles, the first type of negative particles is yellow particles, thesecond type of positive particles is the red particles and the secondtype of negative particles is the white particles.
 8. The layer of claim6, wherein the charges of the first type of positive particles and thefirst type of negative particles are higher than the second type ofpositive particles and the second type of negative particles.
 9. Thelayer of claim 8, wherein the charges of the lower charged particles areless than 50% of the charges of the higher charged particles.
 10. Thelayer of claim 8, wherein the charges of the lower charged particles are5% to 30% of the charges of the higher charged particles.
 11. The layerof claim 8, wherein the charges of the lower charged particles are lessthan 75% of the charges of the higher charged particles.
 12. The layerof claim 8, wherein the charges of the lower charged particles are 15%to 55% of the charges of the higher charged particles.
 13. The layer ofclaim 6, wherein the electrophoretic medium further comprisingsubstantially uncharged neutral buoyancy particles.
 14. The layer ofclaim 13, wherein the neutral buoyancy particles are non-charged.
 15. Adriving method for an electrophoretic fluid comprising four types ofcharged pigment particles dispersed in a solvent or solvent mixture,wherein the four types of charged pigment particles are high positivecharged pigment particles, high negative charged pigment particles, lowpositive charged pigment particles and low negative charged particles;the method comprises: (a) driving a pixel to the color state of one ofthe low charged particles, followed by (b) driving the pixel to thecolor state of high charged particles; wherein the low charged particlesof (a) and the high charged particles of (b) carry opposite chargepolarities.
 16. The driving method of claim 15, further comprising ashaking waveform.
 17. The driving method of claim 15, wherein the highpositive charged particles are black.
 18. The driving method of claim15, wherein the high negative charged particles are yellow.
 19. Thedriving method of claim 15, wherein the low positive charged particlesare red.
 20. The driving method of claim 15, wherein the low negativecharged particles are white.
 21. The driving method of claim 15, whereinthe high positive charged particles are black, the high negative chargedparticles are yellow, the low positive charged particles are red, andthe low negative charged particles are white.
 22. A driving method foran electrophoretic fluid comprising four types of charged pigmentparticles dispersed in a solvent or solvent mixture, wherein the fourtypes of charged pigment particles are high positive charged pigmentparticles, high negative charged pigment particles, low positive chargedpigment particles and low negative charged particles; the methodcomprises: (a) applying a shaking waveform, (b) applying a high drivingvoltage having the same polarity as one type of high charged pigmentparticles to drive to a color state of the high charged pigmentparticle, (c) applying a low driving voltage having the same polarity asone type of low charged pigment particles to drive to a color state oflow charged pigment particles; and (d) applying a high driving voltagehaving the same polarity as the high charged pigment particles to driveto a color state of the high charged pigment particles; wherein the highcharged pigment particles of (d) and the low charged pigment particlesof (c) are oppositely charged and the driving method is DC balanced.